Ultrasonic measuring devices are known for measuring strand-shaped products such as electrical cables, hoses or pipes. A disadvantage of ultrasonic measuring devices is the need to contact to the strand to be measured with a contact medium. The density, temperature and quality of the medium, generally water, strongly influences the measuring result. Moreover, measuring results from such ultrasonic measuring devices depend on the strand temperature, in particular the wall thickness to be measured. Furthermore, known ultrasonic measuring devices do not provide any information on the diameter or out-of-roundness of a strand. Foamed or partially-foamed products have an absorption that is too high for ultrasonic waves and can therefore not be measured with ultrasonic measuring instruments.
So-called terahertz measuring instruments are also known in which a transmitter transmits terahertz radiation at a frequency range of approximately 0.05 to 3 THz; this radiation is reflected by a product to be measured, and the reflected radiation is received by a suitable receiver. With the known terahertz measuring instruments, only distances or wall thicknesses of flat products such as plates are measured, however. The terahertz radiation is focused on the surface of the plate to be measured. Measuring reflection to determine the distance or wall thickness is then comparatively easy. If, however, diameters or wall thicknesses of cylindrical strand products, i.e., cables, pipes or hoses, are to be measured with such measuring instruments, it would be necessary to adjust to the sensor to optimally focus on the surface of the strand to be measured. This holds true in particular when the diameter of the strand to be measured changes. This is associated with significantly complex measurement. Another disadvantage is that for each radius of curvature of a strand to be measured, the algorithms for evaluating the measuring signals must be re-adapted. This is associated with significantly extensive evaluation. In particular with (partially) transparent strands, there is also the problem of evaluating radiation components that are reflected several times.